1. Field of the Invention
The present invention relates to a digital protection relay including a time sync function.
2. Description of the Related Art
FIG. 1 shows a circuit configuration of a conventional digital protection relay.
In FIG. 1, a digital protection relay RY includes an analog input unit 1 that samples an quantity of electricity 1a input from a power system PS at predetermined cycles and converts the analog amount to a digital amount; time sync means 2 that receives a digital quantity of electricity 1b obtained by the analog input unit 1; and a determination unit 3 that compares a digital quantity of electricity 2m output from the time sync means 2 with a determination value and discriminates the presence/absence of a fault in the power system.
The time sync means 2 includes a reception circuit 21 that receives a reception pulse signal tp3 indicative of a reference timing and time data sd3, which are sent from a reference time signal generator 4 (to be described later in detail) via a communication medium L; a pulse detection circuit 22 that generates a reference timing t′ by detecting a digital pulse signal tp4 output from the reception circuit 21; a time calculation unit 23 that calculates a sampling timing 2h on the basis of a time data signal sd4 output from the reception circuit 21 and the reference timing t′ output from the pulse detection circuit 22; and a sampling sync circuit 24 that specifies the sampling timing of the digital quantity of electricity input from the analog input unit 1, on the basis of the sampling timing 2h calculated by the time calculation unit 23.
On the other hand, the reference time signal generator 4 includes a reference timing generating circuit 41 that receives a reception signal 5a received from a positioning system 5 and generates a reference timing t in units of a second; a pulse generating circuit 42 that converts the reference timing t generated by the reference timing generating circuit 41 to a reference timing pulse signal tp1; a time generating circuit 43 that generates time data sd1, such as year/month/day/hour/minute/second, on the basis of the reception signal 5a received from the positioning system 5; and a transmission circuit 44 that transmits to the protection relay RY via the communication medium L the time data sd1 generated by the time generating circuit 43 and the timing pulse signal tp1 generated by the pulse generating circuit 42.
As has been described above, in the conventional digital protection relay, when the reference timing obtained from the reception signal delivered from the positioning system 5 is to be sent to the time sync means 2 in the protection relay, the reference timing is converted to a pulse signal varying in magnitude at the time point of the reference timing and this pulse signal is transmitted, as shown in FIG. 2 by way of example.
However, if the pulse signal input to the reception circuit 21 deteriorates due to noise occurring in the course of communication or degradation in the communication medium L such as transmission/reception elements or cables, the magnitude of the signal varies over the determination threshold value at a timing differing by ΔT from the reference timing, as shown in FIG. 2, and the reference timing may erroneously be recognized.
As a result, the sampling timing of the quantity of electricity data becomes erroneous. In particular, in the case where the presence/absence of a fault in the power system is determined by comparison with quantity of electricity data sampled at different locations in the power system at the same time, a fault, which has not occurred in fact, may be erroneously recognized, or a fault, which has actually occurred, may not be recognized.